10.8 读取位于包中的数据文件

问题¶

Your package includes a datafile that your code needs to read. You need to do this in the most portable way possible.

解决方案¶

Suppose you have a package with files organized as follows:

mypackage/
    __init__.py
    somedata.dat
    spam.py

Now suppose the file spam.py wants to read the contents of the file somedata.dat. To do it, use the following code:

# spam.py
import pkgutil
data = pkgutil.get_data(__package__, 'somedata.dat')

The resulting variable data will be a byte string containing the raw contents of the file.

讨论¶

To read a datafile, you might be inclined to write code that uses built-in I/O functions, such as open(). However, there are several problems with this approach.

First, a package has very little control over the current working directory of the interpreter. Thus, any I/O operations would have to be programmed to use absolute filenames. Since each module includes a __file__ variable with the full path, it’s not impossible to figure out the location, but it’s messy.

Second, packages are often installed as .zip or .egg files, which don’t preserve the files in the same way as a normal directory on the filesystem. Thus, if you tried to use open() on a datafile contained in an archive, it wouldn’t work at all.

The pkgutil.get_data() function is meant to be a high-level tool for getting a datafile regardless of where or how a package has been installed. It will simply “work” and return the file contents back to you as a byte string.

The first argument to get_data() is a string containing the package name. You can either supply it directly or use a special variable, such as __package__. The second argument is the relative name of the file within the package. If necessary, you can navigate into different directories using standard Unix filename conventions as long as the final directory is still located within the package.